LTC3351 Charging Problem

Hello, Sorry I could not read the schematic even if save as picture and zoomed in. To small and fuzzy. Would you include the schematic using the insert link/insert image/video/file? 

Are you able to read the registers via I2C? is it possible to obtain the contents of the sys_status_reg and the monitor_status_reg? When the LTC3351 is failing to charge what is the status of the VINGD pin?

Hello MartyM,

Thank you for your fast reply. Yesterday evening I discovered the problem. It was a solder problem. There was a solder bridge I missed due to assessive solder paste. I build a new prototype pcb and it works fine now. Works as expected.

Though, I have some other questions:

- Is it possible te put the LTC3351 through the reflow proces (oven) twice? For mounting SMD components on the backside or in a two phase proces.

- In the evaluation board schematic there is a 0,1 uF capacitor on the VIN pin witch is not populated. Is there any reason to use a 0,1 uF or larger capacitor on the VIN pin for stability?

- I use a ATMega164PA MCU on the system / board. I coupled the I2C interface of the LTC3351 to the ATMega164PA. On the moment I am not using this interface (I2C) but maybe I will in the future. I used 100K pullup resistors on the I2C pins. Is it good practise to set the relating pins on the ATMega164PA as output and pull them low. Or can I set the pins as input and keep both I2C pins high? What is the best way? I have no pulldown resistors and no room for them on the moment.

Thank you for your help, I really appreciate it.

Best Regards,

Rick Meijers

Hello MartyM,

Thank you for your fast reply. Yesterday evening I discovered the problem. It was a solder problem. There was a solder bridge I missed due to assessive solder paste. I build a new prototype pcb and it works fine now. Works as expected.

Though, I have some other questions:

- Is it possible te put the LTC3351 through the reflow proces (oven) twice? For mounting SMD components on the backside or in a two phase proces.

- In the evaluation board schematic there is a 0,1 uF capacitor on the VIN pin witch is not populated. Is there any reason to use a 0,1 uF or larger capacitor on the VIN pin for stability?

- I use a ATMega164PA MCU on the system / board. I coupled the I2C interface of the LTC3351 to the ATMega164PA. On the moment I am not using this interface (I2C) but maybe I will in the future. I used 100K pullup resistors on the I2C pins. Is it good practise to set the relating pins on the ATMega164PA as output and pull them low. Or can I set the pins as input and keep both I2C pins high? What is the best way? I have no pulldown resistors and no room for them on the moment.

Thank you for your help, I really appreciate it.

Best Regards,

Rick Meijers

Hi Rick, I am glad you found the issue. I do not know about the reflow question. I'll have to get someone from packaging to get that info is someone else does not chime in. 

The DNP cap on VIN was a place holder incase we felt the need to decouple the pin. I do not recall ever needing it for any of the applications or customer boards. 

If there are pullups on the I2C pins they can be left pulled high as inputs or open drain outputs. Pulling them low will not hurt but will use the tiny bit of current in the pullup resistors. These are high impedance inputs so the main thing is that these pins do not float. 

Hello Marty,

Thank you.

Okay, It would be really nice to know about the reflow question for production purpose.

I will try and measure the difference with and without the decouple cap on VIN. For now I left it open and everything seems to be working fine.

Thank you for replying on my I2C question. I will leave them high than, I was just wondering of this would be a problem on long term stability because they are pulled low on the evaluation board.

Best Regards,

Rick Meijers

Hello,

I finally have an answer on the reflow. We guarantee up to 3 reflows.

The demo board has weak pull downs on the SDA & SCL line to keep them at a known state when our USB to I2C adaptor board is not used. Our USB to I2C boards have there own pullup and can be selected to different voltages on the DC590B board.

No long term stability, they just need to be in a known state.

Hello Marty,

Sorry for my late respons. I've been in a rush last months to get the product ready.

Thank you for your answers. I can verify the LCT3351 can go through 3 reflows. I only need 2.

On the moment we are in a test fase. We encoutered a problem with the module. We have been testing for about three weeks now and sometimes the LTC3351 just stops working.

The problem mostly occurs after a shutdown.

The module is build up as follows:

The mosfet switches are controlled by a MCU (Atmel ATMega164PA), the MCU also monitors the input stage and the current to the different loads.

The backup capacitors are 4 x 70 F 2,7 V caps and are build on a seperate PCB that is connected via a short (8 cm) cable.

It is basically a I/O Module for a device that consists of a couple of different units, the main one is a Mini ITX Motherboard.

The problem:

Initially everything works fine. But after a couple of days the module stops working. The Backup caps don't charge anymore and the LTC3351 keeps the input close. After pulling out the backup caps and reconnect them again and connect the main input, everything starts working again..

This happens now and than, sometimes after a couple of days, sometimes after a week or more. We are testing with about 5 machines simultaniously.

Do you have any idea what the problem might be? Will placing the caps on the main PCB help solving this problem? I have not been able to read the LTC3351 registers. Is it possible to read the registers with the DC2026C and the software of the evaluation module? I don't have a display on my PCB so reading out the different LTC registers would be a bit of a issue..

Schematic (DELETED for copyright reasons)

Main PCB, 4 Layer stack.

Supercap PCB, connected with 6 wire cable to main PCB.

Top 3D view of Main PCB, upper left is the connector for the supercaps. On the right side you can see a bit of the DC-DC converter Inductor (SiC461). In the middle the LTC3351.

Layer 2.

Layer 3.

Bottom.

Best Regards,
Rick Meijers

Hi Rick, Thank you for the schematic and layout. This is very helpful. I wonder if resetting the VCC2P5 regulator will solve this. You can try to quickly short the VCC2P5 pin to GND when this happens for troubleshooting purposes to see if this gets you out of the situation. 

The schematic looks good. I do not expect that the short 8cm cable to the caps should cause this problem. It was good that the CAPRTN was Kelvined to the caps. It would be good if CAP4 could be Kelvined also but an extra pin on the connector would be needed and this is not part of the problem. Just for reading ADC accuracy. There is a couple of concerns with the layout. The SW node, bottom and top gate are very long. These traces have fast sharp transients and can radiate noise. The top gate trace should be closely coupled with its return path the SW node. There should be a good GND path from the bottom FET back to the EPAD of the LTC3351 and C45 for the bottom gate return path. It is best to have a solid GND plane on layer 2 if possible to allow good unobstructed GND flow. It looks like there is a good SGND separate from the PGND which is good. They should be connected at a single point. We suggest from pin 11 to the EPAD. I can not tell if they are.

Do you have the DC2026 and LTC3351 software already? I do know if you have the software the DC590B can be fooled by pulling both the EESDA and EESCL lines to EEVCC (pins 11 and 9 to pin 10) with about 4.7kΩ resistor. Then the LTC3351 software can be run separately without using the QuikEval program. I expect this will work the same with the DC2026 but I never tried it.

That's all I got for now.

Thanks, Marty